Bede

In his own words on Numbers

• Numbers 19:2

  Now [Moses] declares that the ashes of the victims (which ought to be taken as a great mystery) are "the sprinkled ashes of a red heifer," which (as the apostle also bears witness) sanctified "those who have been defiled, so that [their] flesh is made clean." He also understands that the sacrament of the Lord's passion, which saves us by purifying us forever, is prefigured in these ashes. Thus the burning of a red heifer designates the actual time and event of Christ's passion, and the burnt ashes which were kept for the cleansing of those who were unclean suggest the mystery of that same passion which has already been completed, by which we are daily purged from our sins.

  On the Tabernacle 2.11

• Numbers 21:6

  The wounds caused by the fiery serpent are the poisonous enticements of the vices, which afflict the soul and bring about its spiritual death. The people were murmuring against the Lord. They were stricken by the serpents' bites. This provides an excellent instance of how one may recognize from the results of an external scourge what a great calamity a person might suffer inwardly by murmuring. In the raising up of the bronze serpent (when those who were stricken beheld it, they were cured) is prefigured our Redeemer's suffering on the cross, for only by faith in him is the kingdom of death and sin overcome. The sins which drag down soul and body to destruction at the same time are appropriately represented by the serpents, not only because they were fiery and poisonous [and] artful at bringing about death but also because our first parents were led into sin by a serpent, and from being immortal they became mortal by sinning. The Lord is aptly made known by the bronze serpent, since he came in the likeness of sinful flesh. Just as the bronze serpent had the likeness of a fiery serpent but had absolutely none of the strength of harmful poison in its members—rather by being lifted up it cured those who had been stricken by the [live] serpents—so the Redeemer of the human race did not merely clothe himself in sinful flesh but entered bodily into the likeness of sinful flesh, in order that by suffering death on the cross in [this likeness] he might free those who believed in him from all sin and even from death itself.

  Homilies on the Gospels 2.18